1. Field of the Invention
The present invention relates to a wiring substrate, and more particularly to a wiring substrate in which a signal wiring layer and a conductor layer are electromagnetically coupled via a dielectric layer for the purpose of transmitting a signal.
2. Description of the Related Art
There are known wiring substrates configured such that a dielectric layer(s), a signal wiring layer(s), a solid conductor layer(s), and other layers are arranged in an alternating fashion. Among such wiring substrates, those involving transmission of a high-frequency signal may assume a strip line structure or a microstrip line structure in which a signal wiring layer and a solid conductor layer are electromagnetically coupled so as to serve as a signal transmission path.
When a wiring substrate is configured such that the dielectric layers and the large-area conductor layers, such as solid conductor layers, are arranged in an alternating fashion, a number of circular or rectangular through holes are often formed in the conductor layers at predetermined intervals, such as in a lattice pattern, or meshed so as to directly connect the upper and lower dielectric layers through the through holes (or mesh).
There are many conventional reasons for connecting the upper and lower dielectric layers directly together through the through holes provided in the conductor layer. The first reason is that the bonding strength between dielectric layers, which are formed from ceramics (e.g., alumina) and/or resins (e.g., epoxy resin), is greater than that between a dielectric layer and a conductor layer, which is formed from metal (e.g., tungsten, molybdenum, copper, or silver). Therefore, the direct bonding of the dielectric layers through the through holes of the conductor layer increases the strength of the wiring substrate. The second reason is that a conductor layer has poorer gas release properties than a dielectric layer, which is especially important during firing, curing, thermal treatment, and other processes performed during manufacture of the wiring substrate. Therefore, during the course of such treatments or processes, a conductor layer having a large area is likely to hinder diffusion of gas released from an underlying dielectric layer or conductor layer. This gas release phenomenon may generate blisters between a dielectric layer and a conductor layer in contact with the dielectric layer, which reduces the bonding strength between the layers of the wiring substrate.
For these reasons, the conventional wisdom is to provide a conductor layer, which is electromagnetically coupled with a signal wiring layer, with a number of through holes to maintain the bonding strength with a dielectric layer and to prevent the generation of blisters.
Although the conventional wiring substrate is generally thought to be acceptable, it is not without shortcomings. Namely, when a conductor layer (which is to be electromagnetically coupled with a signal wiring layer) has a number of through holes formed therein or is meshed as shown in FIG. 6, for example, the through holes are located at portions of the conductor layer that face the signal wiring layers. That is, and with reference to FIG. 6, when a conductor layer 2 having through holes 2B and signal wiring layers 4 (shown in phantom) are arranged together, the conductor layer 2 and the signal wiring layers 4 overlap. However, the through holes 2B and the signal wiring layers 4 also overlap.
The signal wiring layers 4 differ in characteristic impedance between the case where the conductor layer 2 and the signal wiring layers 4 overlap and the case where the through holes 2B and the signal wiring layers 4 overlap. Thus, when a single signal wiring layer 4 is examined for characteristics along its longitudinal direction, the signal wiring layer 4 shows local variations in characteristics, such as characteristic impedance. As a result, a signal transmitted through the signal wiring layer 4 suffers reflection, distortion, and other similar defects that potentially result in delays or errors in the signal transmission.
The present invention overcomes the shortcomings associated with conventional wiring substrates. An object of the present invention is to provide a wiring substrate that exhibits improved bonding strength and reliability, and in which variations in characteristics, such as characteristic impedance, are restrained with respect to signal wiring layers.
To achieve the above objects, the present invention provides a wiring substrate having a layered structure comprising a conductor layer having a plurality of through holes formed therein, a dielectric layer in contact with the conductor layer; and a signal wiring layer positioned in opposition to the conductor layer via the dielectric layer. The signal wiring layer is adapted to transmit a signal. The conductor layer includes (1) a wiring correspondence region corresponding to the signal wiring layer as projected onto the conductor layer in a thickness direction of the conductor layer, and (2) a wiring noncorrespondence region, which is the remaining portion of the conductor layer not including the wiring correspondence region. The through holes are arranged in the wiring noncorrespondence region of the conductor layer.
In the wiring substrate of the present invention, the through holes are formed in the wiring noncorrespondence region of the conductor layer remaining after removal of the wiring correspondence region. Since the signal wiring layer is electromagnetically coupled with the wiring correspondence region, in which no through holes are provided, the signal wiring layer is not prone to characteristic variations in electromagnetic coupling as observed along the longitudinal direction of the signal wiring layer. Thus, local variations in characteristics, such as characteristic impedance, can be restrained with respect to the signal wiring layer.
Notably, no particular limitation is imposed on the configuration of the wiring substrate so long as the signal wiring layer and the conductor layer are arranged in layers in such a manner as to face each other via the dielectric layer. The present invention includes, for example, the following configurations: a conductor layer underlies a signal wiring layer via a dielectric layer; and a signal wiring layer underlies a conductor layer via a dielectric layer. The present invention further includes a configuration, such as a strip line structure, in which a signal wiring layer is sandwiched between two conductor layers.
Preferably, the above-described wiring substrate is configured such that at least one of the through holes are formed in a side portion of the wiring noncorrespondence region, which is located in the vicinity of and along the wiring correspondence region.
As mentioned previously, the configuration in which the through holes are not formed in the wiring correspondence region restrains variations in characteristic impedance and like characteristics. Also, in the wiring substrate of the present invention, the through holes are formed in a side portion of the wiring noncorrespondence region which extends along the wiring correspondence region. Thus, regions in the vicinity of the through holes (i.e., the regions including the wiring correspondence region) exhibit, among other properties, good gas release properties and strong bonding between the upper and lower dielectric layers, thereby preventing generation of blisters in the wiring correspondence region of the conductor layer, and thus imparting high reliability to the wiring substrate.
Preferably, the above-described wiring substrate is configured such that the through holes formed in the side portion are elongated in a direction substantially parallel to the length direction of the signal wiring layer rather than along a direction perpendicular to the length direction.
In the wiring substrate of the present invention, the through holes have an elongated shape; thus, the through holes formed in the side portion located in the vicinity of the wiring correspondence region can assume a relatively large opening area as compared with circular or square through holes. Therefore, although through holes are not formed in the wiring correspondence region, regions in the vicinity of the through holes (i.e., the regions including the wiring correspondence region) can exhibit, among other properties, particularly good gas release properties, thereby reliably preventing generation of blisters or occurrence of a like defect.
The shape of an elongated through hole is not particularly limited so long as the through hole is longer along the direction substantially parallel to the length direction of the signal wiring layer than along the direction perpendicular to the length direction. Specific examples of the form of an elongated through hole include an elongated circle, an oval, and an elongated rectangle.
In another aspect of the present invention, the wiring substrate includes a conductor layer having a plurality of through holes formed therein, a dielectric layer in contact with the conductor layer, and a plurality of signal wiring layers positioned in opposition to the conductor layer via the dielectric layer. The signal wiring layers are adapted to transmit signals. The conductor layer includes (1) a plurality of wiring correspondence regions corresponding to the plurality of signal wiring layers as projected onto the conductor layer in the thickness direction of the conductor layer, and (2) a wiring noncorrespondence region, which is the remaining portion of the conductor layer after removal of the wiring correspondence regions. The through holes are arranged in the wiring noncorrespondence region of the conductor layer. The signal wiring layers include parallel wiring portions extending in parallel with one another and located adjacent to one another. The wiring correspondence regions include parallel wiring correspondence portions corresponding to the parallel wiring portions. The wiring noncorrespondence region includes parallel-wiring-gap correspondence portions respectively interposed between the parallel wiring correspondence portions. The through holes arranged in the parallel-wiring-gap correspondence portions are elongated in a direction substantially parallel to the length direction of the parallel wiring portions of the signal wiring layers rather than along a direction perpendicular to the length direction.
In the wiring substrate of the present invention, the through holes are formed in the wiring noncorrespondence region of the conductor layer. Since the signal wiring layer is electromagnetically coupled with the wiring correspondence regions, in which through holes are absent, the signal wiring layer is not prone to characteristic variations in electromagnetic coupling along the longitudinal direction of the signal wiring layers. Thus, local variations in characteristics, such as characteristic impedance, can be restrained with respect to the signal wiring layers.
When a wiring substrate is configured such that signal wiring layers comprise parallel wiring portions extending in parallel with one another, parallel-wiring-gap correspondence portions of a conductor layer are each interposed between parallel wiring correspondence portions; thus, limitations are imposed on the position where through holes are to be formed and the width of a region where through holes are to be formed. Accordingly, circular or square through holes may fail to provide a required size (opening area) when formed in the parallel-wiring-gap correspondence portions.
According to the present invention, the through holes arranged in the parallel-wiring-gap correspondence portions are elongated. The width of the individual parallel-wiring-gap correspondence portions limits the through holes to be formed in the parallel-wiring-gap correspondence portions with respect to a dimension as measured along the direction perpendicular to the length direction of the parallel wiring portions (or the parallel wiring correspondence portions of the wiring correspondence regions), whereas no limitation is imposed on a dimension as measured in the direction parallel to the length direction of the parallel wiring portions. The elongated through holes of the present invention can assume a relatively large opening area in the parallel-wiring-gap correspondence portions adjacent to the parallel wiring correspondence portions. Accordingly, regions in the vicinity of the through holes (i.e., the regions including the parallel wiring correspondence regions) can exhibit, among other properties, particularly good gas release properties and strong bonding between the upper and lower dielectric layers, thereby reliably preventing the generation of blisters or the occurrence of similar defects.
Preferably, the above-described wiring substrate is configured such that elongated through holes located on opposite sides of a certain parallel wiring correspondence portion are staggered along the length direction of the parallel wiring portions.
When elongated through holes on opposite sides of a certain parallel wiring correspondence portion are arranged in a laterally aligned condition with respect to a certain parallel wiring correspondence portion, the aligned through holes define a narrow gap therebetween (i.e., the through holes narrow a path of current flowing through the conductor layer, raising a problem in that the resistance of the conductor layer increases locally).
By contrast, the wiring substrate of the present invention is configured such that elongated through holes located on opposite sides of a certain parallel wiring correspondence portion are staggered (i.e., adjacent elongated through holes sandwiching a corresponding parallel wiring corresponding portion are arranged in a staggered manner in the length direction). Thus, the gap defined between staggered through holes on opposite sides of a parallel wiring correspondence portion is not narrowed, thereby lowering resistance at such portions of the conductor layer.
Preferably, the above-described wiring substrate is configured such that a gap is formed along the above-mentioned length direction between adjacent through holes located on opposite sides of a parallel wiring correspondence portion.
In the wiring substrate of the present invention, a gap is formed along the above-mentioned length direction between the staggered, elongated through holes. That is, elongated through holes are formed in the conductor layer such that no portion of the conductor layer is interposed between through holes located on opposite sides of a parallel wiring correspondence portion as observed along the length direction.
In yet another aspect of the present invention, the wiring substrate having a layered structure includes a conductor layer having a plurality of through holes formed therein, a dielectric layer in contact with the conductor layer, and a plurality of signal wiring layers positioned in opposition to the conductor layer via the dielectric layer. The signal wiring layers are adapted to transmit a signal. The conductor layer includes (1) a plurality of wiring correspondence regions corresponding to the plurality of signal wiring layers as projected onto the conductor layer in the thickness direction of the conductor layer, and (2) a wiring noncorrespondence region, which is the remaining portion of the conductor layer after removal of the wiring correspondence regions. The through holes are arranged in the wiring noncorrespondence region of the conductor layer. The signal wiring layers include (1) first parallel wiring portions extending in parallel with one another such that a first gap is formed between the adjacent first parallel wiring portions, and (2) second parallel wiring portions extending in parallel with one another while forming a predetermined angle with respect to the first parallel wiring portions and such that a second gap greater than the first gap is formed between the adjacent second parallel wiring portions. The wiring correspondence regions include (1) first parallel wiring correspondence portions corresponding to the first parallel wiring portions, and (2) second parallel wiring correspondence portions corresponding to the second parallel wiring portions. The wiring noncorrespondence region includes (1) first parallel-wiring-gap correspondence portions that are interposed between the first parallel wiring correspondence portions, and (2) second parallel-wiring-gap correspondence portions each being interposed between the second parallel wiring correspondence portions. The through holes that are arranged in the first parallel-wiring-gap correspondence portions are first through holes that have a shape that is elongated in a direction substantially parallel to the first length direction along the first parallel wiring portions of the signal wiring layers rather than along a direction perpendicular to the first length direction. The through holes that are arranged in the second parallel-wiring-gap correspondence portions are second through holes that have a shape that is elongated in a direction substantially parallel to the second length direction along the second parallel wiring portions of the signal wiring layers rather than along a direction perpendicular to the second length direction. Also, the second through holes have an opening area greater than that of the first through holes.
In the wiring substrate of the present invention, the through holes are formed in the wiring noncorrespondence region of the conductor layer remaining after removal of the wiring correspondence regions. Since the signal wiring layer is electromagnetically coupled with the wiring correspondence regions, in which through holes are absent, the signal wiring layer is not prone to characteristic variations in electromagnetic coupling as observed along the longitudinal direction of the signal wiring layers. Thus, local variations in characteristics, such as characteristic impedance, can be restrained with respect to the signal wiring layers.
For a wiring substrate configured to have first and second parallel wiring portions, the first parallel-wiring-gap correspondence portions and the second parallel-wiring-gap correspondence portions of the conductor layer are respectively interposed between the first parallel wiring correspondence portions or between the second parallel wiring correspondence portions. Thus, limitations are imposed on the position where the through holes are to be formed and the width of a region where through holes are to be formed. Accordingly, circular or square through holes may fail to provide a required size (opening area) when formed in the first parallel-wiring-gap correspondence portions and the second parallel-wiring-gap correspondence portions.
According to the present invention, the through holes arranged in the first parallel-wiring-gap correspondence portions and the second parallel-wiring-gap correspondence portions are implemented as the first elongated through holes or the second elongated through holes, to thereby assume a relatively large opening area as compared with through holes in a circular or like shape. Therefore, regions located in the vicinity of the first elongated through holes and the second elongated through holes (i.e., the regions including the first parallel wiring correspondence regions and the second parallel wiring corresponding regions) can exhibit, among other properties, particularly good gas release properties and strong bonding between the upper and lower dielectric layers, thereby reliably preventing the generation of blisters and the occurrence of other similar defects.
The first parallel wiring correspondence portions correspond to the first parallel wiring portions arranged at intervals of a first gap, whereas the second parallel wiring correspondence portions correspond to the second parallel wiring portions arranged at intervals of a second gap greater than the first gap. Accordingly, the second parallel-wiring-gap correspondence portions are arranged at intervals wider than those at which the first parallel-wiring-gap correspondence portions are arranged. Therefore, if the second elongated through holes assume the same size as the first elongated through holes, an increase in interval is accompanied by a relative reduction in opening area per unit of area (opening percentage), with the result that blisters tend to be generated due to impaired gas release properties.
According to the present invention, the second elongated through holes arranged in the second parallel-wiring-gap correspondence portions assume an opening area greater than that of the first elongated through holes, whereby a reduction in opening percentage is prevented. Therefore, in either the first parallel-wiring-gap correspondence portions or the second parallel-wiring-gap correspondence portions, the generation of blisters and the occurrence of other similar defects can be further reliably prevented in the vicinity of the first elongated through holes or the second elongated through holes (including the first parallel wiring correspondence portions and the second parallel wiring correspondence portions).
The above and other features of the invention including various and novel details of construction and combination of parts will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular wiring substrate embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.